Broad spectrum optical supercontinuum source

ABSTRACT

An optical supercontinuum radiation source for generating a broad optical supercontinuum from pump radiation having a wavelength in the range 900 nm to 1200 nm includes a microstructured optical fibre and a pump laser adapted to generate pump radiation for pumping the microstructured optical fibre. The fibre can have a Δ (“delta”) value of greater than 0.3, the core region of the fibre can support a plurality of modes at the pump wavelength, and the cladding region can comprise at least two air holes extending along the length of fibre wherein the ratio of the diameter (d) of the air holes to their pitch (Λ) is greater than 0.6. The fibre can comprise a zero dispersion wavelength (ZDW) within ±200 nm of said pump wavelength.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of application Ser. No.12/866,738, filed Aug. 8, 2010 and entitled “A Source of OpticalSupercontinuum Radiation.” Application Ser. No. 12/866,738 claimspriority to International PCT Patent Application PCT/GB09/50122, bearingan international filing date of Feb. 6, 2009 and entitled “A Source ofOptical Supercontinuum Radiation.” PCT/GB09/50122 claims priority toPatent Application GB 0802356.6, filed Feb. 8, 2008. The foregoingapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a source of optical supercontinuumradiation and particularly, but not exclusively, to a microstructuredoptical fibre for the generation of blue-enhanced supercontinua.

BACKGROUND

Supercontinuum generation refers to the formation of broad continuousspectra by the propagation of a high power, relatively narrow spectralbandwidth laser radiation through a nonlinear medium. The continuum isproduced via a number of nonlinear effects including self-phasemodulation, Raman scattering and four-wave mixing. Initial experimentswith supercontinuum generation used very high power lasers (mJ or μJ perpulse) with ultrashort pulses, typically <1 ps, focused into glass,sapphire and even water. The extremely high power of many megawatts wassufficient to create broad bandwidth spectra, typically spanning anoctave or more (i.e. from a certain wavelength to twice thatwavelength). However, the high power required meant that large, bulklaser systems were needed and often damaged the nonlinear media. Inaddition, the supercontinuum typically had an unstable and irregulartransverse profile. The breakthrough was provided with the advent ofspecialised optical fibres providing high confinement within the corei.e. a high power density, which can propagate over long interactionlengths and therefore generate supercontinua at relatively lower laserpowers and in a single transverse mode.

Optical fibres are commonly based on silica glass and in the spectralregion beyond 1300 nm in which the material dispersion of silica glassis anomalous, optical fibres can be designed and fabricated to have agroup velocity dispersion which is normal or anomalous, with a zerodispersion at any given wavelength over a wide range. It is not possiblehowever, to move the zero dispersion wavelength of a conventional silicastep-index, single-mode optical fibre to wavelengths less than 1270 nm,namely the zero dispersion wavelength of bulk silica.

The zero dispersion wavelength (ZDW) is an important parameter in thegeneration of supercontinua as, to-date, the widest spectra are producedwhen the pump wavelength, i.e. the laser wavelength that is coupled intothe fibre, is close to the ZDW. In microstructured fibres or, morecommonly, photonic crystal fibres (PCFs) however, it is possible toshift the ZDW of single mode fibres to much shorter wavelengths tothereby enable other laser sources having different wavelengths to beutilised as pump sources in the generation of supercontinua.

PCFs are most commonly formed from silica material and comprise a solidsilica core region surrounded by a cladding region, which comprises anarray of hexagonal-close-packed air filled holes in a silica backgroundmatrix. The air-silica cladding material creates an effective refractiveindex which is less than the refractive index of the core region andthus permits the guidance of light within the core by a variation of thetraditional mechanism of total internal reflection.

PCFs are characterised largely by the diameter of the core, the ratio ofthe diameter (d) of the air holes in the cladding to their spacing, orpitch (Λ), and the ratio of the difference between the refractive indexof the core and the effective refractive index of the cladding region,to the refractive index of the core, often referred to as “delta” or Δ.PCFs can be manufactured to support a single confined mode (i.e. singlemode fibre) or a plurality of confined modes (multimode fibre),respectively. More significantly, PCFs can also be manufactured to beendlessly single mode (ESM), such that ESM fibres can support only onemode for all wavelengths.

U.S. Pat. No. 6,097,870, hereinafter '870, and the correspondingscientific paper (Optics Letters, Vol. 25, No. 1, Jan. 1, 2000—“VisibleContinuum Generation in Air-Silica Microstructure Optical Fibres withAnomalous Dispersion at 800 nm”), discloses a microstructured opticalfibre which is pumped at approximately 800 nm to generate opticalsupercontinuum in the range 390 nm-1600 nm. The fibre disclosed in '870is inherently single mode at the pump wavelength which is the preferencein the art given that single mode fibres have the best characteristicsin terms of diffraction, transverse profile, stability and smallest spotsize of the transmitted beam. Furthermore, single mode fibres do notexhibit modal dispersion, can carry more information than multimodefibres and are better at retaining the fidelity of each pulse over longdistances. However, while the supercontinuum generated in the '870patent spans from as low as 400 nm in the blue region to the infra-redregion of the spectrum, the supercontinuum described in '870 can only beobtained using 800 nm pump wavelengths, and at this wavelength typicalpump sources are unreliable and unfeasibly expensive for manyapplications.

There are two basic designs of PCF which have been used in the art forthe generation of supercontinuum. One is the high-Δ fibre similar tothat disclosed in the '870 patent. This type of fibre has the advantageof strong beam confinement giving a high nonlinear coefficient and azero dispersion at short wavelengths.

The second type of fibre is the ESM fibre comprising relatively smalldiameter air holes in the cladding. This type of fibre can produce azero dispersion wavelength around 1060 nm or 800 nm, but the nonlinearcoefficient is not as strong. However, it has the advantage of beingintrinsically single mode. The trend in the art is to use ESM fibres forpumping at wavelengths of 1060 nm.

It is an object of the present invention to provide a source of opticalsupercontinuum radiation for generating an optical supercontinuum havingblue-enhanced spectral components.

SUMMARY

In accordance with the present invention there is provided a source ofoptical supercontinuum radiation, the source comprising amicrostructured optical fibre and a pump laser adapted to generatelasing radiation at a pump wavelength,

-   -   the microstructured optical fibre comprising a core region and a        cladding region which surrounds the core region;    -   the fibre comprising a zero dispersion wavelength within ±200 nm        of said pump wavelength, wherein the fibre can support a        plurality of modes at said pump wavelength; and    -   the pump laser is adapted to launch said lasing radiation at        said pump wavelength into the core region of the microstructured        optical fibre to excite the fundamental mode of the fibre.

The microstructured fibre of the present invention is thus not singlemode at the pump wavelength. In contrast, the disclosure of '870 impliessingle mode at the pump wavelength.

The microstructured optical fibre modifies the group index dispersion atinfra-red wavelengths such that the group index at these wavelengths isidentical to that at blue and ultraviolet wavelengths of less than 390nm and thereby provides for an enhanced blue generation in thesupercontinuum.

Preferably, the ZDW of the fibre is less than 1000 nm.

It is preferred that the pump wavelength is separated from the ZDW ofthe fibre by at least 10 nm and more preferably by at least 20 nm.Offsetting the pump wavelength from the ZDW of the fibre in this waymaximises the bandwidth of the supercontinuum.

The core region of the microstructured fibre preferably comprises afirst refractive index and the cladding region comprises a secondeffective refractive index such that the Δ-value is greater than 0.03.

The microstructured optical fibre preferably comprises a core ofsubstantially circular cross-section having a diameter in the range 1-5μm. The cladding region preferably comprises at least two capillary airholes having substantially the same diameter and which extendsubstantially along the length of the fibre. Preferably, the ratio ofthe diameter of said air holes to their separation is greater than 0.6.The microstructured fibre is preferably formed substantially of silica.

The pump laser may comprise a mode-locked laser, a Q-switched laser, again-switched laser, or a master oscillator power amplifier. The pumplaser preferably comprises a fibre laser. The pump wavelength ispreferably in the range 900 nm to 1200 nm, most preferably in the range1000 nm to 1100 nm, and is desirably in the range 1050 nm to 1070 nm.The pump laser most preferably comprises a mode locked fibre laseroperating at substantially 1064 nm. The pump laser may be arranged togenerate linearly polarised lasing radiation.

Preferably, the fibre exhibits anomalous dispersion at the pumpwavelength.

The specific processes involved in the generation of the supercontinuumof the present invention are strongly influenced by the variation of thegroup index across the transparency window of the PCF. The variation ofthe group index with wavelength generally takes the form of a skewed“U”, with rapidly decreasing group index (normal group-velocitydispersion) moving from shorter wavelengths, a zero-crossing in thegroup velocity dispersion (GVD) typically around the pump wavelength at1060 nm, and increasing group index (anomalous dispersion) towardslonger wavelengths. The microstructured optical fibre of the source ofoptical supercontinuum radiation of the present invention has avariation of group index with wavelength in which the “U” rises moresteeply on the short wavelength side than at longer wavelengths, becauseof the strong material dispersion of silica at these shorterwavelengths. A frequency-shifting soliton propagating in theanomalous-dispersion (infra-red) regime effectively traps blue radiationpropagating with the same group index on the other arm of the “U” in apotential well and scatters the blue radiation to shorter wavelengths ina cascaded four-wave mixing process.

The microstructured optical fibre preferably has a group index curveadapted to match the group index at wavelengths of greater than 2000 nmto the corresponding group index at a wavelength of less than 400 nm.

The long and short wavelength side of the group index dispersion, namelythe extent of the supercontinuum, is limited by the very rapidly risingabsorption of the material as one moves beyond 2.5 μm, which istypically exacerbated in PCF's by the presence of OH— ions which have anabsorption band at around 2.4 μm. The group index variation of themicrostructured optical fibre of the source of optical supercontinuumradiation of the present invention enables shorter blue wavelengths tobe accessed due to the higher rate of change of group index withwavelength in the infra-red wavelength region.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a source of opticalsupercontinuum radiation according to a first embodiment of the of thepresent invention;

FIG. 2 is a scanning electron micrograph image of a cross-section of thelarge core, high-Δ fibre of the source of optical supercontinuum of FIG.1;

FIG. 3 is a graphical representation of the full spectrum output fromthe source of optical supercontinuum radiation of FIG. 1;

FIG. 4 is a scanning electron micrograph image of a cross-section of anESM fibre;

FIG. 5 is a graphical representation of the group index curves for thelarge core high-Δ fibre of FIG. 2 and the ESM fibre of FIG. 4, plottedas a function of wavelength (the insert shows the short wavelength edgesof the supercontinuum generated from the high-Δ fibre and the ESMfibre);

FIG. 6 is a graphical representation of the modelled variation of groupindex with wavelength, for bulk silica, an ESM PCF and for a strand ofsilica surrounded by air;

FIG. 7 a is a graphical representation of short wavelength continuumedges for high-Δ fibres of a source of optical supercontinuum radiationin accordance with a second embodiment of the present invention, havingcore diameters of 4.7 μm, 4.4 μm and 4.2 μm; and

FIG. 7 b is a graphical representation of modelled group index curvesfor high-Δ fibres of the source of optical supercontinuum radiation inaccordance with the second embodiment of the present invention, havingcore diameters of 4.7 μm, 4.4 μm and 4.2 μm.

DETAILED DESCRIPTION

Referring to FIG. 1 of the drawings and in accordance with a firstembodiment of the present invention, the source of opticalsupercontinuum radiation 10 comprises a microstructured optical fibre20, fabricated using the stack and draw method, and a pump laser 30. Themicrostructured optical fibre 20 comprises a core substantially 4.7 μmin diameter and a large index contrast (Δ˜0.03) between the core 20 aand cladding regions 20 b. The pitch was measured to be substantially3.7 μm and the ratio d/Λ was measured to be substantially 0.77. An SEMimage of a cross-section of the fibre 20 is shown in FIG. 2.

The pump laser 30 is a Q-switched microchip laser 30 operable togenerate infrared lasing pulses of 600 ps to 1 ns duration at a 10 kHzrepetition rate, and centred at 1064 nm, which were coupled into thecore 20 a of a 10 m length of the fibre 20 via an objective lens 40, andthe input power of the pump laser was varied by means of a neutraldensity (ND) filter wheel 50 that was placed in front of the fibreinput, as shown in FIG. 1. The output power was measured using a powermeter (not shown). The polarisation state of the coupled laser beam canbe controlled by means of additional polarisation elements (not shown).

The fibre 20 is multimode at the pump wavelength, however, only thefundamental mode is excited in generating the supercontinuum. Thegenerated supercontinuum is also almost exclusively in the fundamentalmode. The short wavelength output (350-1750 nm) was collected by anextra fibre (not shown) and recorded on an optical spectrum analyser(not shown), while the long wavelength edge of the continuum (900-2550nm) was collected in a short, straight length of single mode fibre (notshown) and recorded on an IR spectrometer (not shown). The pump peakswere filtered out using a long pass filter (cut-off wavelength 1600 nm)to prevent multi-order interference in the measurement. The full opticalspectrum output from the high-Δ fibre 20 is shown in FIG. 3; the pump at1064 nm is clearly visible as the peak 60.

In order to provide a suitable comparison of the data, the infraredlasing pulses were also coupled into a 10 m length of ESM (d/Λ=0.43,Λ=3.0 μm) fibre 70, and the output was recorded using the power meter(not shown) and the optical spectrum analyser (not shown). An SEM imageof a cross-section of the ESM fibre 70 is shown in FIG. 4.

Spectra were recorded for four different output powers using each fibre20, 70 and the short and long wavelength edges identified, by selecting.a point at a fixed value, 10, 15 or 20 dB less than a feature whichappeared in all the spectra for either the long or short wavelengthedges. The group index curves for the high-Δ fibre 20 and the ESM fibre70 are plotted as functions of wavelength in FIG. 5.

It is evident that the high-Δ fibre 20 creates a broader continuum thanthe ESM fibre 70, and extends further into the blue region of thespectrum than the continuum generated from the ESM fibre 70. Theagreement is good, that is, the lines joining the short and longwavelength edges are almost horizontal on the plot. This gives strongsupport to the concept of group-index matching between the longest andshortest wavelengths being a limiting factor in blue and ultravioletsupercontinuum generation.

Comparing the group index of bulk silica 100 with the theoretical groupindex for an ESM PCF 110 and for a strand of silica 120 surrounded byair, as shown in FIG. 6, it is seen that although at short wavelengthsthe three curves are almost indistinguishable, the behaviour at longwavelengths is very different.

The waveguide dispersion causes a steeper increase in the group index(increasing the anomalous dispersion) which matches the index at aspecific infrared wavelength to significantly shorter wavelengths in theultraviolet. As this group-index matching is what is required for bluelight generation, it is apparent that a strand of silica surrounded byair would generate shorter wavelengths than an ESM PCF. The fibre 20used in the source of optical supercontinuum radiation 10 of the presentinvention approximates a strand of silica surrounded by air.

The fibre group index curve can be modified to push the continuumfurther into the near ultraviolet, by reducing the core size. However,this also shifts the zero dispersion wavelength away from the pump.

According to a second embodiment of the invention, a series of threehigh-Δ fibres were drawn, the fibre 20 as described above and twoidentical fibres with different outer diameters and hence core sizes,which were measured to be ˜4.4 μm 130 and ˜4.2 μm 140 (the outerdiameters of the three fibres 20, 130, 140 are 100 μm, 95 μm and 90 μm).Each of these fibres was separately used to generate supercontinuum inaccordance with the arrangement illustrated in FIG. 1. The fibres 130,140 are multimode at the pump wavelength as is fibre 20, and each fibre,20, 130, 140 was pumped in an identical manner to that previouslydescribed with only the fundamental mode being excited in the individualfibres. The spectra output from each fibre were recorded directly on anoptical spectrum analyser. The normalized short wavelength edges of thespectra for each fibre 20, 130, 140 and the modelled group index curvesare shown in FIGS. 7( a) and (b), respectively.

Referring to FIG. 7, it is evident that the individual spectra do indeedextend to shorter wavelengths, namely less than 390 nm for the smallercores. The primary mechanism for supercontinuum generation on the longwavelength side of the pump is Raman shifting solitons, which areunaffected by the proximity of the pump to the fibre zero dispersionpoint. On the short wavelength side of the pump however, new frequenciesare initially generated by other processes such as four-wave mixing(prior to being shifted to deeper blue frequencies by group indexmatched solitons), which require the pump to be in close proximity tothe zero dispersion wavelength of the fibre. As the fibre pitch isdecreased to steepen the infrared edge of the group index curve the zerodispersion points are also shifted away from the pump thereby reducingthe amount of new frequencies generated.

It will be appreciated that the pump laser may alternatively comprise amode-locked fibre laser, a gain switched laser or a master oscillatorpower amplifier (MOPA).

The sources of optical supercontinuum radiation of the present inventionenable the creation of a truly “white” light source by including thewavelength region 350-400 nm in the generated spectrum. The entirespectrum is generated in the fundamental mode of the fibre. The fibredesign used, with large air holes to modify the group index profile ofthe fibre, enables the sources of optical supercontinuum described togroup-index-match long-wavelength-edge (infrared) radiation to shorterwavelengths in the blue/ultra-violet than has previously been possible.The shorter wavelength band which is incorporated in the supercontinuagenerated by the sources described will allow new applications to beunlocked from compact supercontinuum sources.

1. An optical supercontinuum radiation source for generating a broadoptical supercontinuum from pump radiation having a wavelength in therange 900 nm to 1200 nm, comprising: a microstructured optical fibrecomprising a core region and a cladding region which surrounds the coreregion; a pump laser adapted to generate pump radiation at a pumpwavelength and to pump said microstructured optical fibre with said pumpradiation; said pump wavelength being in the range 900 nm to 1200 nm;said core region of said fibre comprising a first refractive index andsaid cladding region comprising an effective refractive index such thatsaid fibre has a Δ (“delta”) value of greater than 0.3, where Δ refersto the ratio of the difference between the refractive index of the coreregion and the effective refractive index of the cladding to therefractive index of the core region; said core region supporting aplurality of modes at said pump wavelength; said cladding regioncomprising at least two air holes extending along the length of fibreand wherein the ratio of the diameter (d) of said air holes to theirpitch (Λ) is greater than 0.6; and wherein said fibre comprises a zerodispersion wavelength (ZDW) within ±200 nm of said pump wavelength. 2.The source of optical supercontinuum radiation according to claim 1,wherein the ZDW of the fibre is less than 1000 nm.
 3. The source ofoptical supercontinuum radiation according to claim 2 wherein said pumpwavelength is in the range 1000 nm to 1100 nm.
 4. The source of opticalsupercontinuum radiation according to claim 1 wherein said pumpwavelength is in the range 1000 nm to 1100 nm.
 5. The source of opticalsupercontinuum radiation according to claim 4 wherein themicrostructured optical fibre exhibits anomalous dispersion at said pumpwavelength.
 6. The source of optical supercontinuum radiation accordingto claim 1 wherein said pump wavelength is in the range 1050 nm to 1070nm.
 7. The source of optical supercontinuum radiation according to claim1 wherein said pump wavelength is substantially 1064 nm.
 8. The sourceof optical supercontinuum radiation according to claim 1 wherein thegenerated optical supercontinuum is almost exclusively in thefundamental mode.
 9. The source of optical supercontinuum radiationaccording to claim 1 wherein the microstructured optical fibre exhibitsanomalous dispersion at said pump wavelength.
 10. The source of opticalsupercontinuum radiation according to claim 1 wherein said pump laserpumps said microstructured fibre such that only the fundamental mode ofthe microstructured fibre is excited.
 11. The source of opticalsupercontinuum radiation according to claim 1 wherein said ZDW isseparated from said pump wavelength by at least 10 nm.
 12. The source ofoptical supercontinuum radiation according to claim 1 wherein said ZDWis separated from said pump wavelength by at least 20 nm.
 13. The sourceof optical supercontinuum radiation according to claim 12 wherein saidZDW is less than 1000 nm.
 14. The source of optical supercontinuumradiation according to claim 12 wherein said microstructured opticalfibre exhibits anomalous dispersion at said pump wavelength.
 15. Thesource of optical supercontinuum radiation according to claim 1 whereinsaid microstructured fibre is formed substantially of silica.
 16. Thesource of optical supercontinuum radiation according to claim 1 whereinsaid source is adapted and constructed such that the opticalsupercontinuum radiation includes the wavelength region 350 nm-400 nm inthe generated spectrum.
 17. The source of optical supercontinuumradiation according to claim 1 wherein said source is adapted andconstructed such that the generated optical supercontinuum radiationextends to wavelengths less than 390 nm.
 18. The source of opticalsupercontinuum radiation according to claim 1 wherein said source isadapted and constructed such that the generated optical supercontinuumradiation includes the wavelength of 450 nm as well as the wavelength of2100 nm.
 19. The source of optical supercontinuum radiation according toclaim 1 wherein said microstructured fibre is a single clad opticalfibre.
 20. The source of optical supercontinuum radiation according toclaim 1, wherein said microstructured optical fibre has a group indexcurve adapted to match the group index at wavelengths of greater than2000 nm to a corresponding group index at a wavelength of less than 400nm.
 21. The source of optical supercontinuum radiation according toclaim 1, where said core region has a diameter between 4.2 and 4.7microns.